In the field of computer networking, cut-through forwarding, also known as cut-through switching, is a switching method for packet switching systems in which a network switch starts forwarding a frame (or packet) before the whole frame has been received by the network switch. Such a forwarding operation is performed typically as soon as the destination address has been processed. In this manner, cut-through forwarding enables the latency through the switch to be significantly reduced. The use of cut-through forwarding is an important feature of packet orientated deterministic automation systems. Furthermore, the implementation of such systems is expanding in the industrial market, and the technique is increasingly finding its way into solutions for the home, medical and automotive applications.
Cut-through forwarding systems typically require tight control over the latency of a switch (which is the delay between data being received by the switch and that data subsequently being transmitted (forwarded on) by the switch) and jitter (which is the variance in time periods between reception and the transmission of the same frame) in order to ensure deterministic behaviour and scalability. Typically, such latency/jitter requirements differ between different cut-through ‘modes’ (for example, between different packet switching protocols). For example, such cut-through modes might include, by way of example, Ethernet protocols such as EtherCAT (Ethernet for Control Automation Technology), ProfiNET, Ethernet/IP, DLR (Device Level Ring) or a cut through switch for IP traffic. Control over latency and jitter is of particular importance for cut-through forwarding modes that involve Ethernet frames and the like, in which frames are unpredictably spaced and may be seconds apart or back to back, unlike, say, Voice over IP (VoIP) which has predictable frame spacing. In order for a cut-through switch to be competitive in the market place, it must be capable of supporting such deterministic behaviour and scalability across multiple cut-through switching modes.
Conventionally, tight control of latency and jitter is provided by way of dedicated hardware blocks that are arranged to meet specific latency and jitter requirements. The use of such dedicated hardware blocks on a single device leads to a relatively expensive and inflexible solution. In particular, in order for a given switch to be able to meet the requirements of more than one cut-through mode, a separate, dedicated hardware block is required for each cut-through mode, thereby resulting in a significant increase in cost, power consumption and real estate requirements for the switch.